In oil and gas well drilling operations it may be useful to determine the types of fluid being produced downhole from a formation, such as hydrocarbons in the form of natural gas or oil, and water. Fluids such as water are a common byproduct of petroleum production and can cause difficulties such as reduced hydrocarbon production and increased water disposal costs. Determination and detection of water production may allow the well operator to take appropriate remedial actions can enhance the economic life of individual wells and the petroleum reservoir as a whole.
The fluid being produced may be made up of more than one type of fluid the overall fluid composition may be described in terms of holdup. Holdup is the fraction of a particular fluid present in an interval of pipe or borehole. In addition to composition, the fluid being produced may also be described in terms of flow rate. In multiphase flow, for example, each fluid may move at a different speed due to several factors. The holdup of a particular fluid is not the same as the proportion of the total flow rate due to that fluid, also known as its cut. To determine in-situ flow rates, it is necessary to measure the holdup and velocity of each fluid, the sum of the holdups of the fluids present being unity.
To determine the fluids being produced and the percentage make-up of those fluids in a given volume, a production log operation may be performed using a fluid identification tool like hydro or density sensor to measure the hold of each fluid at a particular depth. For water, the holdup can often be derived from measurements of fluid capacitance and density of the production fluid. Devices that measure capacitance however are less accurate in fluids with high (more than 40%) water cuts because of the nonlinear nature of the tool response above 40% water cut. Devices that measure density are likewise sensitive to increased water salinity which increases conductivity. Density devices also begin to lose resolution in heavy oil, where the density of oil approaches that of water. Recent microwave, radio frequency, and infrared devices used to determine water holdup also experience decreased accuracy in high water content environments. Most microwave probes are also affected by changes in water salinity and oil density.